Add readme

Author: nagisa

README.md

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+Barebones wrapper around libFuzzer runtime library.
+
+The CPP parts are extracted from llvm git repository with `git filter-branch`.
+
+# How to use
+
+“Manual” usage of this library looks like this:
+
+```
+$ cargo new --bin fuzzed
+$ cd fuzzed
+
+$ tail Cargo.toml -n2 # add libfuzzer-sys dependency
+[dependencies]
+fuzzer-sys = { path = "../libfuzzer-sys" } # or something
+
+$ cat src/main.rs
+#![no_main]
+extern crate fuzzer_sys;
+
+#[export_name="LLVMFuzzerTestOneInput"]
+pub fn go(data: *const u8, size: isize) -> i32 {
+    // fuzzed code goes here
+    0
+}
+
+$ cargo rustc -- -C passes='sancov' -C llvm-args='-sanitizer-coverage-level=3' -Z sanitizer=address -Cpanic=abort
+$ ./target/debug/fuzzed # runs fuzzing
+```
+
+Nice wrappers incoming soon